Biogeosciences [B]

B52A MCC:2008 Friday

Recent Advances in Soil Ecology and Their Contribution to Understanding Soil Carbon Dynamics III

Presiding:J Curiel Yuste, University of California; L Misson, University of California

B52A-01 INVITED

Deconvolving Soil CO2 Efflux at Three Temporal Scales

* Davidson, E A (edavidson@whrc.org) , Woods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543 United States
Savage, K E (savage@whrc.org) , Woods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543 United States

Soil respiration is a combination of plant and microbial processes that respond to climatic drivers at a variety of temporal and spatial scales. Deconvolving the soil respiration signal into several component processes and at different time scales may help us understand the controls on each. We used correspondence analysis and regression analysis of half-hourly soil CO2 efflux measurements using automated chambers at the Harvard Forest of central Massachusetts to investigate the climatic drivers of soil respiration at three time scales: diel cycles, synoptic weather patterns, and seasonality. The diel cycle corresponded to diel variation in soil temperature. The amplitude of the diel cycle was proportional to the daily mean flux. In other words, the diel variation is greatest under warm and moist conditions. Synoptic weather patterns bring rainfall events that generally lead to a pulse of CO2 production and efflux, which is revealed by correspondence between CO2 efflux and soil water content at time scales of 4-14 days. The magnitude of the wet-Up pulse was correlated with the number of preceding dry days and the size of the precipitation event. Once the effects of diel cycles and wetting events are partitioned out of the data, the remaining trend is seasonal and was correlated with soil temperature. However, this seasonal pattern was not symmetric, but rather showed a steeper incline during the spring than decline during the autumn. This seasonal hysteresis is consistent with a stronger contribution of respiration of growing roots during the spring than during the autumn. By assuming that wintertime CO2 efflux is primarily from microbial respiration and that the microbial response to seasonal temperature variation follows an exponential fit with a Q10 of about 2.5, then the difference between this modeled seasonal pattern of microbial respiration and the deconvolved seasonal pattern of total CO2 efflux yields a rough estimate of the seasonal pattern of root respiration, which was skewed toward the spring, as would be expected for temperate forests.

B52A-02

Quantifying the combined effects of elevated atmospheric CO2 and nutrient amendments on subsurface CO2 production in a southern Loblolly pine plantation using Inverse Methods

* Daly, E (edaly@pratt.duke.edu) , Department of Civil and Environmental Engineering, Duke University, Hudson Hall, Durham, NC 27708 United States
* Daly, E (edaly@pratt.duke.edu) , Nicholas School of the Environment and Earth Sciences, LSRC, Durham, NC 27708 United States
Porporato, A (amilcare@duke.edu) , Department of Civil and Environmental Engineering, Duke University, Hudson Hall, Durham, NC 27708 United States
Porporato, A (amilcare@duke.edu) , Nicholas School of the Environment and Earth Sciences, LSRC, Durham, NC 27708 United States
Oren, R (ramoren@duke.edu) , Nicholas School of the Environment and Earth Sciences, LSRC, Durham, NC 27708 United States
Katul, G (gaby@duke.edu) , Department of Civil and Environmental Engineering, Duke University, Hudson Hall, Durham, NC 27708 United States
Katul, G (gaby@duke.edu) , Nicholas School of the Environment and Earth Sciences, LSRC, Durham, NC 27708 United States

Subsurface $CO_2$ production ($S$), one of the largest $CO_2$ sources to the atmosphere, has been the subject of intense studies because of its potential role in amplifying global warming. Projected warming trends associated with rise in atmospheric $CO_2$ can lead to higher soil temperature and greater $S$ thereby completing the positive feedback.Surprisingly, the individual and combined effects of elevated atmospheric $CO_2$ and nitrogen deposition rates on $S$ remain poorly understood, especially in forested ecosystems. Field studies on the effects of elevated atmospheric $CO_2$ on $S$ are mixed with several studies reporting an increase in $S$ because of an increase in root biomass and enhanced microbial activity, while others reporting only transient changes. On the other hand, several field experiments documented a clear suppression of $S$ with increased nitrogen amendments. Resolving the combined effects of elevated atmospheric $CO_2$ and nitrogen amendments on $S$ is complicated by an intricate balance between various physical and biological processes. To begin confronting this problems, frequent {\it in situ} measurements of root and microbial respiration at multiple soil depths and at the same location must be conducted. Here, we quantify the joint and individual effects of elevated atmospheric $CO_2$ and nutrient amendments on $CO_2$ production rates in the soil pore spaces within the root-zone using a combination of field measurements and inverse modeling* across a wide range of soil moisture states. The field experiment utilizes the Free Air $CO_2$ Enrichment facility in which $30$ m rings enriched with $CO_2$ are also fertilized. The inverse model calculations use an array of small solid-state $CO_2$ sensors for measured spatial concentration distributions along with measured soil moisture and soil temperature to estimate gas-phase $CO_2$ diffusivity. Implications to below ground carbon cycling and their linkages to alterations in root-water uptake patterns due to elevated $CO_2$ and $N$ are also discussed.

B52A-03

Partitioning Soil Respiration Between Autotrophic and Heterotrophic Components in a Mature Boreal Black Spruce Stand

* Gaumont-Guay, D (dgguay@interchange.ubc.ca) , Biometeorology and Soil Physics Group, University of British Columbia, 266B-2357 Main Mall , Vancouver, BC V6T 1Z4 Canada
Black, T A (andrew.black@ubc.ca) , Biometeorology and Soil Physics Group, University of British Columbia, 266B-2357 Main Mall , Vancouver, BC V6T 1Z4 Canada
Barr, A G (Alan.Barr@EC.GC.CA) , Climate Research Branch, Meteorological Service of Canada, 11 Innovation Blvd., Saskatoon, SK S7N 3H5 Canada
Jassal, R S (rachhpal@interchange.ubc.ca) , Biometeorology and Soil Physics Group, University of British Columbia, 266B-2357 Main Mall , Vancouver, BC V6T 1Z4 Canada
Morgenstern, K (kai.morgenstern@ubc.ca) , Biometeorology and Soil Physics Group, University of British Columbia, 266B-2357 Main Mall , Vancouver, BC V6T 1Z4 Canada
Nesic, Z (zoran.nesic@ubc.ca) , Biometeorology and Soil Physics Group, University of British Columbia, 266B-2357 Main Mall , Vancouver, BC V6T 1Z4 Canada

A root-exclusion experiment conducted in mature boreal black spruce stand (125 year-old) in Saskatchewan, Canada, from September 2003 to December 2004 allowed the partitioning of soil respiration between autotrophic (roots, mycorrhizae and decomposers associated with the rhizosphere) and heterotrophic (free-living organisms) components using continuous automated chamber measurements of soil CO2 efflux. The exclusion of live roots caused a 25% reduction in soil respiration three weeks after the application of the treatment in September 2003, which suggested a strong link between tree photosynthesis and belowground respiration processes. Annual estimates of autotrophic and heterotrophic respiration were 324 and 230 g C m$^{-2}$ y$^{-1}$ in 2004, accounting for 53 and 38% of soil respiration, respectively, after correcting for the decomposition of roots killed by trenching (78 g C m$^{-2}$ y$^{-1}$). The remainder (57 g C m$^{-2}$ y$^{-1}$) originated from live-moss respiration. Over the course of the year, there was a gradual transition from heterotrophic to autotrophic-dominated respiration with three distinctive phases: (1) autotrophic respiration was negligible during winter when the trees were dormant; (2) heterotrophic respiration dominated soil respiration during the shoulder periods of April-May and October-November when soil temperature was low; (3) autotrophic respiration exceeded heterotrophic respiration from mid-July to mid-September when soil temperature was high and trees were active. Both components of respiration increased exponentially with soil temperature during the growing season but autotrophic respiration showed greater temperature sensitivity than heterotrophic respiration. The replenishment of soil water following spring snowmelt induced a sustained increase in heterotrophic respiration. Pulses in autotrophic respiration were observed during summer following large rainfalls that were attributed to rhizosphere priming effects. After normalizing autotrophic respiration for the seasonal variation in soil temperature, it was found to be strongly correlated with tree photosynthesis. Analysis showed a lagged response with a maximum correlation for 15-25 days Tree photosynthesis also exerted a strong control on autotrophic respiration at the diurnal time scale with a lagged response of approximately 12 hours. These results suggest that the characterization of the soil temperature and water regimes is not sufficient to describe accurately the seasonal and diurnal variations in soil respiration and its components. Models need to incorporate the controls of aboveground photosynthetic production, photosynthate allocation and phloem transport on soil respiration.

B52A-04

Interactions Between Temperature and Nutrient Availability in Mediating Microbial Respiration in High Arctic Polar Semi-desert Soils

* Holland, K J (holland@lifesci.ucsb.edu) , University of California, Santa Barbara, Dept. Ecology, Evolution, and Marine Biology University of California, Santa Barbara, CA 93106-9610 United States
Sullivan, P (anpfs@uaa.alaska.edu) , University of Alaska, Anchorage, Environment and Natural Resources Institute University of Alaska, Anchorage 707 A St., Anchorage, AK 99501 United States
Wallenstein, M (wallenstein@lifesci.ucsb.edu) , University of California, Santa Barbara, Dept. Ecology, Evolution, and Marine Biology University of California, Santa Barbara, CA 93106-9610 United States
Arens, S (setharens@yahoo.com) , University of Alaska, Anchorage, Environment and Natural Resources Institute University of Alaska, Anchorage 707 A St., Anchorage, AK 99501 United States
Schimel, J P (schimel@lifesci.ucsb.edu) , University of California, Santa Barbara, Dept. Ecology, Evolution, and Marine Biology University of California, Santa Barbara, CA 93106-9610 United States
Welker, J M (afjmw1@uaa.alaska.edu) , University of Alaska, Anchorage, Environment and Natural Resources Institute University of Alaska, Anchorage 707 A St., Anchorage, AK 99501 United States

Field respiration measurements in high arctic polar semi-desert in northern Greenland suggest a divergence in respiration rates of microbial communities in fertilization treatments at temperatures above $4C. We hypothesized that this divergence could be attributed to either greater temperature responsiveness of microbial communities in nitrogen fertilized treatments, or to increased substrate availability in nitrogen fertilization treatments at higher temperatures. Microbial respiration responses to labile substrate addition were equal across fertilization treatments, suggesting that microbial communities had similar temperature sensitivities. To determine whether substrate availability differed between fertilization treatments, we measured 13CO2 of respiration at four temperatures. With increased temperature, rates of CO2 efflux increased and isotopic signatures of respired carbon became lighter, suggesting increasing turnover of more recalcitrant C at higher temperatures. Respiration of nitrogen fertilized soils had lighter 13CO2 signatures than ambient soils, suggesting that nitrogen might increase turnover of more recalcitrant soil carbon. These data suggest the divergence in CO2 efflux in the nitrogen fertilization treatments could be mediated by increasing availability of recalcitrant carbon.

B52A-05

Integrating Microbial Community Composition With Biogeochemical Carbon and Nitrogen Dynamics: Examples From Lignin and Polyphenol Decomposition

* Waldrop, M (mwaldrop@usgs.gov) , US Geological Survey, 345 Middlefield Rd, MS 962, Menlo Park, CA 94025 United States
Zak, D R (drzak@umich.edu) , The University of Michigan, School of Natural Resources and Environment, Ann Arbor, MI 48109 United States
Blackwood, C (cbwood@umich.edu) , The University of Michigan, School of Natural Resources and Environment, Ann Arbor, MI 48109 United States
Harden, J (jharden@usgs.gov) , US Geological Survey, 345 Middlefield Rd, MS 962, Menlo Park, CA 94025 United States

Biogeochemical models conceptually utilize box and arrow diagrams to explain the rates of carbon cycling in soil. Within these models, labile, intermediate, and recalcitrant pools of carbon are linked to each other, to respiration, and dissolved organic carbon (DOC) flux using parameterized rate functions. These models have often been successful at predicting carbon cycling rates, but they often have to be parameterized to new environmental conditions. This may occur in part because biogeochemical models do not explicitly include the underlying biological mechanisms controlling decomposition. Biogeochemical models may be improved by advances in our understanding the distribution, biomass, and activity of decomposer functional groups. It is especially useful to understand the dynamics of decomposer functional groups and enzyme systems that breakdown recalcitrant soil carbon such as lignin and condensed polyphenolics. Quantitative PCR (QPCR) is an advance in molecular biology that can target decomposer functional groups and functional genes that holds promise for understanding the landscape-level variability in microbial communities controlling the flow and fate of carbon. Here we provide examples of how the abundance and distribution of soil fungi in grassland, temperate and boreal forests predicts the enzymatic capacity of the soil community to decompose recalcitrant soil C. Moreover, the abundance of soil fungi has important implications for the response of decomposers to soil N availability.

B52A-06

Autotrophic and Heterotrophic Controls over Winter Soil Carbon Cycling in a Subalpine Forest Ecosystem

* Monson, R K (Russell.Monson@colorado.edu) , University of Colorado, Department of Ecology and Evolutionary Biology, Boulder, CO 80309 United States
* Monson, R K (Russell.Monson@colorado.edu) , University of Colorado, Cooperative Institute for Research in Environmental Science, Boulder, CO 80309 United States
Scott-Denton, L E (Laura.Scott@colorado.edu) , University of Colorado, Department of Ecology and Evolutionary Biology, Boulder, CO 80309 United States
Lipson, D A (dlipson@sciences.sdsu.edu) , San Diego State University, Department of Biology, San Diego, CA 92182 United States
Weintrub, M N (Michael.Weintraub@colorado.edu) , University of Colorado, Department of Ecology and Evolutionary Biology, Boulder, CO 80309 United States
Rosenstiel, T N (Todd.Rosenstiel@colorado.edu) , University of Colorado, Department of Ecology and Evolutionary Biology, Boulder, CO 80309 United States
Schmidt, S K (Steve.Schmidt@colorado.edu) , University of Colorado, Department of Ecology and Evolutionary Biology, Boulder, CO 80309 United States
Williams, M W (Mark.Williams@colorado.edu) , University of Colorado, Department of Geography and Institute for Arctic and Alpine Research, Boulder, CO 80309 United States
Burns, S P (Sean.Burns@colorado.edu) , University of Colorado, Department of Ecology and Evolutionary Biology, Boulder, CO 80309 United States
Burns, S P (Sean.Burns@colorado.edu) , National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80305 United States
Delany, A E (delany@ucar.edu) , National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80305 United States
Turnipseed, A A (turnip@ucar.edu) , National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80305 United States

Studies were conducted at the Niwot Ridge Ameriflux site to understand wintertime soil carbon cycling and its control over ecosystem respiration. Wintertime respiration in this ecosystem results in the loss of 60-90% of the carbon assimilated the previous growing season. Thus, an understanding of the controls over winter carbon cycling is required to understand controls over the annual carbon budget. Trees were girdled to prevent the transport of photosynthates to the rhizosphere. In plots with non-girdled trees a large mid-winter pulse of sucrose was observed to enter the soil. In plots with girdled trees, no sucrose pulse was observed. Trees of this ecosystem are not photosynthetically active during the winter, leading us to conclude that the sucrose pulse is due to the death of fine roots that had accumulated sucrose the previous autumn. The sucrose pulse is potentially utilized by a novel winter community of microbes. Using DNA fingerprinting we discovered that the dominant isolates from the winter soils were from Jathinobacter, whereas the summer isolates were from Burkholderia. The winter community was capable of high rates of respiration and exponential growth at low temperatures, whereas the summer community was not. Our winter observations also indicated high activity of N-acetyl-â-glucosaminidase, one of the principal enzymes involved in chitin degradation. The presence of such high chitinase activities implicates decomposing fungal biomass as a principle source of CO2 beneath the snow pack. Using a novel in situ, beneath-snow CO2 measurement system, we observed unprecedented Q10 values for winter respiration, being 98 and 8.44 x 10$^{4}$ for the soil next to tree boles or within the open spaces between trees, respectively. These high Q10 values are likely the result of fractional changes in the availability of liquid water below 0C and responses of microbial biomass to changes in the liquid water fraction. Using six-years of eddy covariance data, we showed that interannual variation in winter ecosystem respiration is positively correlated to interannual variation in the spring snow depth. Years with a with a deeper spring snow pack exhibited higher soil temperatures, and concomitantly higher soil respiration rates. Given the recently reported decadal-scale trend in decreasing snow pack in the Western U.S., which is coupled to warm climate anomalies, our observations indicate the potential for higher wintertime soil carbon sequestration due to lower winter ecosystem respiration rates in subalpine forests. Our studies of processes beneath the winter snow pack demonstrate that contrary to previous assumptions, winter biogeochemical processing of soil organic matter is an important component of ecosystem carbon budgets. Despite low temperatures and an inactive plant rhizosphere, winter microbial communities and exoenzymes appear to be active, carbon substrates appear to be in relatively high abundance and soil respiration rates appear to be sensitive to seasonal and interannual winter climate variability.

B52A-07

Nitrogen Alters Fungal Communities in Boreal Forest Soil: Implications for Carbon Cycling

* Allison, S D (allisons@uci.edu) , Departments of Ecology and Evolutionary Biology and Earth System Science, University of California, Irvine, CA 92697 United States
Treseder, K K (treseder@uci.edu) , Departments of Ecology and Evolutionary Biology and Earth System Science, University of California, Irvine, CA 92697 United States

One potential effect of climate change in high latitude ecosystems is to increase soil nutrient availability. In particular, greater nitrogen availability could impact decomposer communities and lead to altered rates of soil carbon cycling. Since fungi are the primary decomposers in many high-latitude ecosystems, we used molecular techniques and field surveys to test whether fungal communities and abundances differed in response to nitrogen fertilization in a boreal forest ecosystem. We predicted that fungi that degrade recalcitrant carbon would decline under nitrogen fertilization, while fungi that degrade labile carbon would increase, leading to no net change in rates of soil carbon mineralization. The molecular data showed that basidiomycete fungi dominate the active fungal community in both fertilized and unfertilized soils. However, we found that fertilization reduced peak mushroom biomass by 79%, although most of the responsive fungi were ectomycorrhizal and therefore their capacity to degrade soil carbon is uncertain. Fertilization increased the activity of the cellulose-degrading enzyme beta-glucosidase by 78%, while protease activity declined by 39% and polyphenol oxidase, a lignin-degrading enzyme, did not respond. Rates of soil respiration did not change in response to fertilization. These results suggest that increased nitrogen availability does alter the composition of the fungal community, and its potential to degrade different carbon compounds. However, these differences do not affect the total flux of CO2 from the soil, even though the contribution to CO2 respiration from different carbon pools may vary with fertilization. We conclude that in the short term, increased nitrogen availability due to climate warming or nitrogen deposition is more likely to alter the turnover of individual carbon pools rather than total carbon fluxes from the soil. Future work should determine if changes in fungal community structure and associated differences in substrate utilization will also affect total carbon fluxes over longer time scales.

B52A-08

Advective Transport of CO2 in Permeable Media Induced by Atmospheric Pressure Fluctuations

* Massman, W J (wmassman@fs.fed.us) , USDA Forest Service, Rocky Mountain Research Station 240 West Prospect, Fort Collins, CO 80526 United States

Pressure fluctuations at the earth's surface are caused by a variety of naturally occuring atmospheric phenomena. Such pressure fields force air to move in and out of soils, snowpacks, and other permeable media and therby influence the exchange rate of many trace gases between the underlying permeable substrate to the atmosphere. Consequently, the uptake or release of trace gases from soils and snowpacks is a combination of molecular diffusion and advective flows caused by surface pressure fluctuations. First, this paper develops a physically-based analytical model that describes the effects of natural pressure pumping on CO2 profiles and fluxes within a layered medium. The model is a two-layered model, which assumes that each layer has uniform, but different, physical properties with a CO2 source term located in either the upper or lower layer. The pressure forcing, modeled as plane wave in time and the horizontal direction, has an amplitude that varies in the vertical direction as described by an analytical solution to the diffusion equation. The CO2 response is decomposed into a steady-state diffusion solution and a plane wave solution of the advective-diffusive equation, which also has an amplitude that varies vertically. The model is formulated for both dispersive and non-dispersive media. In the case of a dispersive medium, the dispersion coefficient [m2/s] is expressed in terms of the horizontal wave number, the amplitude of the pressure forcing at the upper surface, and the vertical structure and dispersivity [m] of the medium. Second, the model is applied to the case of a snowpack with an underlying [snow-covered] soil. Meadow and forest CO2 amounts, sampled beneath an approximately meter deep (steady state) snowpack at a subalpine site in southern Rocky Mountains of Wyoming (USA) between December 19, 2000 and February 8, 2001, are observed to vary by nearly 200 ppmV over periods ranging from 4 - 15 days. With the aid of the physically-based model inferences are made about the nature of the physical properties of both the forcing mechanism and the snowpack that contribute to these periodic variations in undersnow CO2. Results are consistent with the hypothesis that the undersnow CO2 is being driven by advective flows induced by pressure fields created when the wind interacts with the local aerodynamic roughness elements (nearby mountain peaks, forest edges, snowdrifts). Non-harmonic spectral and cospectral techniques indicate that the wind modulates the low frequency temporal dynamics of the undersnow CO2. Whereas, comparisons of the modeled and observed time lag between the surface forcing and the response of the undersnow CO2 suggest that site topography determines the horizontal structure of the wind (surface pressure) forcing. Further results also suggest that the snowpack is at best a weakly dispersive medium. Finally, because the model includes a CO2 source term in the soil underlying the snowpack, other findings suggest that both the wintertime CO2 fluxes emanating from the snowpack and the soil respiration rates may vary significantly between a meadow soil and a forest soil at this site. During the period covered by this study the meadow soil displayed higher respiration fluxes in the wintertime than the forest soil.